An attention is paid to such a channel aggregation that available channels existing in various frequency bands are bound and transmitted in order to increase a communication capacity. In a wireless LAN system such as IEEE802.11ac, a cellular system such as LTE-Advanced, a white space wireless system such as IEEE802.11af, a fifth generation mobile communication system (5G system), or the like, international standard specifications related to the channel aggregation have completely been settled or are being settled and their studies and development are being progressed.
In the foregoing systems, an (OFDM: Orthogonal Frequency Division Multiplexing) or an (OFDMA: Orthogonal Frequency Division Multiple Access) as an OFDM which enables a multiple access by the multiuser is adopted as a modulation system. In such an OFDM(A) system, complex modulation symbols are mapped to a number of subcarriers arranged so as to intersect at right angles with each other, a time-series signal is derived by inverse discrete Fourier transforming such a complex modulation symbol group, and transmission OFDM(A) symbols for wireless transmission are formed. In order to suppress an influence of a propagation delay wave, a guard interval (GI) is inserted into the transmission OFDM(A) symbols. On the reception side, after the OFDM(A) symbols of the number corresponding to a discrete Fourier transformation duration were extracted, by discrete Fourier transforming them and performing proper propagation path estimation and equalization, a complex modulation symbol group of the transmitted subcarriers is derived, and by executing a receiving process such as a demodulation or the like, transport data is reproduced.
Although the OFDM(A) largely contributes to improvement of the transmission capacity of the communication system owing to its high frequency use efficiency, out-of-band radiation electric power (i.e., out-of-band emission) is high and there is a problem about an electric power consumption efficiency. Particularly, in a close-packed terminal environment, a white space secondary use environment, or the like, it is very important to suppress the out-of-band radiation electric power. In order to suppress the out-of-band radiation electric power, a low pass filter (LPF) is effective. As an LPF-based new physical layer signal waveform, a UF-OFDM (Universal-filtered OFDM), an FBMC (Filter bank multicarrier), or the like has been proposed. However, for example, in a wireless LAN or an LTE (Long term evolution) as one of the 4G systems, as a guard bandwidth in a band, a width of up to about 0.25 MHz is left on both sides. In order to install an LPF for sufficiently suppressing the out-of-band radiation electric power within such a narrow guard band, a large calculation amount scale is necessary.
In the wireless LAN, 5G system, or the like, a system called a channel aggregation (or carrier aggregation or channel bonding) in which a plurality of continuous or discontinuous transmission frequency channels are bound and transmitted as one system has been introduced or its introduction is being examined. Since an effective transmission band is widened by binding and transmitting a plurality of channels, the transmission capacity as a system increases.
Fundamentally, most of the communication systems so far use such a form that a frequency channel is used in a band which has unconditionally been allocated. However, in the channel aggregation, such a possibility that channels of separate bands under different circumstances are bound and used like a spectrum mask or the like occurs. Even in the case of performing the aggregation of adjacent or neighboring channels, for example, in white space communication as a communication form in which a license band is secondarily used, there is also a case where a request spectrum mask changes in dependence on whether or not the adjacent channel is occupied by the primary user and there is also a case where the channels under the different spectrum mask conditions are bound and used. When a plurality of channels under the different spectrum mask conditions are used, a necessity of designing a plurality of LPFs which accomplish out-of-band radiation suppressing performance which satisfies the conditions in each channel occurs. According to those LPFs, since a filter length in a time-domain varies in dependence on design conditions, in order to satisfy a time-domain signal length decided by a certain system format, a transmission symbol generating process becomes complicated or difficult.
With respect to an OFDM(A) signal transmitter for transmitting an OFDM(A) signal without using any LPF, the inventors of the present invention has proposed a (Universal time-domain windowed OFDM; UTW-OFDM) system which can be efficiently operated even at the time of applying such a channel aggregation that even in the case of performing OFDM(A) wireless communication in an operation frequency band such as a white space or the like in which transmission spectrum specifications are severe, an out-of-band radiation is sufficiently suppressed and a plurality of channels in the same band or in different bands are bound and transmitted (refer to Patent Literature 1). The method whereby the out-of-band radiation electric power of OFDM(A) is suppressed by a time-domain window is mainly introduced to the wireless LAN. However, since the conventional requested spectrum mask is not severe, in many time-domain windows which have often been installed at present, a possibility that an influence is exerted at the time of using the adjacent channel in the close-packed terminal environment, white space secondary use environment, or the like is high. In the UTW-OFDM system, by increasing a very short window length in the time-domain window which has often been used in the related art to the same length as a symbol length at most, a very large out-of-band radiation electric power suppressing effect can be obtained. In the invention, since the out-of-band radiation suppressing process is executed by the time-domain window, fundamentally, it can be realized by a multiplying process and becomes a convolution process in the LPF based method as a frequency-domain process, and a problem of an increase in calculation amount can be solved. Even in the case where the channels under the different spectrum mask conditions are bound and transmitted, such a design that a (Universal Time-domain Window; UTW) of a different kind can be individually and easily applied to the time-domain signal allocated to each channel is used. Therefore, an efficient channel aggregation which can cope with the different channel operating conditions while satisfying a signal time length format which is required for the system can be performed.